Agricultural equipment with automatic steering control requires a constantly updated heading measurement for the navigation system. Traditional GPS receivers produce a heading measurement with every GPS update, but the error of the heading measurement increases dramatically as vehicle speed falls below 1 mph and approaches zero. This is because the heading measurement is based on the change in vehicle position from one GPS update to the next, and as speed approaches zero, the change in vehicle position during the GPS sample period becomes small in relation to GPS measurement error. Discussion of such things as automatic steering systems, including GPS (GNSS) sub-systems, can be found in such U.S. patents and published applications as: U.S. Pat. No. 9,141,111B2 to inventors Webber et al., U.S. Pat. No. 7,860,628B2 to inventor Lange, and US2014/0214275A1 to inventors Miller et al., each of which is incorporated by reference herein in its entirety.
Where a single antenna GPS is used, heading measurement becomes very noisy as the speed drops below about 1 mph. Although a dual antenna GPS system may be used to provide a more accurate heading measurement at low and zero speed, the equipment adds significant cost. Examples of multiple antenna GNSS positioning sub-systems can be found at US2009/0121932A1 to inventor Whitehead and U.S. Pat. No. 8,140,223B2 to assignee Hemisphere GGS LLC, each of which is incorporated by reference herein in its entirety.
What is needed are methods, apparatuses, and systems which allow for accurate heading measurements, even at low speeds.